All the living organisms (plants and animals) respond and react to changes in the environment around them. The changes in the environment to which the organisms respond and react are called stimuli (singular of stimuli is stimulus). The living organisms show response to stimuli such as light, heat, cold, sound, smell, taste, touch, pressure, pain, water, and force of gravity, etc.
The response of organisms to a stimulus is usually in the form of some movement of their body part. For example, if a man touches a very hot utensil accidentally, he quickly pulls his hand away from the hot utensil. Here, heat is the stimulus and the man reacts by moving his hand away from the hot utensil. Similarly, when the sun is bright, we close our eyes. In this case, light is the stimulus and we reacting by closing the eyes.
When we are frightened by a dog, we run away as fast as we can. Here, fear (of dog) is the stimulus and we react by running away. If we prick an earthworm with a needle, then the earthworm withdraws (moves back). In this case, pain (produced by pin prick) is the stimulus and the earthworm reacts by withdrawing.
We know that a sunflower always faces the sun. Here, sunlight is the stimulus and sunflower reacts by bending (or moving) towards the sun. We eat food when we are hungr(and need energy).
In this case, hunger is the stimulus and we react by eating food. From the above discussion we conclude that the reaction to stimuli is a characteristic property of the living organisms. Another word which is also used in place of ‘reaction’ is ‘response’. So, we can also say that the response to stimuli is a characteristic property of the living organisms.
Both, plants and animals react (or respond) to various stimuli around them. But the method of reacting to stimuli is not similar in plants and animals. They react to stimuli in different ways. For example, plants bend towards light but animals do not bend towards light. The animal Amoeba reacts to the presence of food by moving towards the food particle.
Similarly, Amoebae tend to aggregate (collect together) in moderately warm water which is their reaction to the stimulus called heat. Amoeba and other protozoal react to the mechanical obstacles by avoiding them. We find that the Amoeba (which is an animal) can react to different stimuli in different ways.
The animals can react to stimuli in many different ways because they have a nervous system and an endocrine system involving hormones. The plants, however, react to stimuli in a very limited way. This is because the plants do not have a nervous system like the animals have. The plants use only the hormones for producing reaction to external stimuli.
From all the above examples we conclude that when a stimulus acts on our body, then we react (or 1 respond) in a manner which is in the best interest of our body. The reaction (or response) which we give to the stimulus involves many organs of our body.
It is, therefore, necessary that all the concerned organs should work with one another in a systematic manner so as to produce the required reaction. In other words, the various organs should co-operate with one another to provide proper reaction to the stimulus.
The working together of the various organs of an organism in a systematic manner so as to produce a proper response to the stimulus is called coordination. We will now discuss the control and coordination in plants, animals and human beings, one by one. Let us start with control and coordination in plants.
The plants do not have a nervous system and sense organs like eyes, ears, or nose, etc., like the animals, but they can still sense things. The plants can sense the presence of stimuli like light, gravity, chemicals, water, and touch, etc., and respond to them. The plants can sense things like light, gravity, chemicals, water, and touch, etc., by the action of hormones in them.
The stimuli like light, gravity, chemicals, water, and touch, etc., are called environmental changes. So, we can also say that the plants coordinate their behaviour against environmental changes by using hormones. The hormones in plants do not act the same way as in animals.
The hormones in plants coordinate their behaviour by affecting the growth of a plant. And the effect on growth of the plant can result in the movement of a part of the plant like shoot (stem) or root, etc.
Animals use both nervous system and hormones for coordination of their activities. Plants have no nervous system, so plants use only hormones for coordination. Thus, the reaction (or response) of plants to different stimuli like light, gravity, chemical substances, water, and touch etc., is due to the effect of hormones.
Please note that animals can respond quickly because they have a nervous system. Plants cannot respond quickly because they have no nervous system. The plants respond to various stimuli very slowly by growing. So, in most of the cases, the
From the above discussion we conclude that the function of control and coordination in plants is performed by the chemical substances called plant hormones. Please note that the plant hormones are also called phytohormones (‘phyto’ means ‘plant’). Before we discuss the various types of plant hormones, we should know the meanings of ‘dormancy’ and ‘breaking of dormancy’.
A resting, inactive condition in which metabolism almost stops is called dormancy. The seed of a plant is inactive or dormant. It has dormancy. A seed must have certain conditions like water, warmth, air and hormones to break dormancy and germinate to form a seedling (which then grows into a plant).
Another part of a plant having dormancy is the bud. The bud is a young, undeveloped shoot of a plant which on breaking dormancy can form a branch, a leaf or a flower depending on its position in the plant. The breaking of dormancy of a bud also requires certain plant hormones. Keeping these points in mind, we will now discuss the various types of plant hormones.